1. Field of the Invention
The present invention relates to sensors, e.g., in-flight sensors on board an aircraft, and more particularly to detecting clouds with bimodal droplet size distributions.
2. Description of Related Art
The detection of airborne liquid droplets and their classification according to droplet size is an important function of an in-flight sensor on board an aircraft. Large droplets, though much fewer in number than smaller droplets, present a greater threat to aircraft icing. The ability to detect bimodal droplet size distributions in icing clouds is important for an aircraft icing conditions sensor.
The Optical Ice Detector (OID) is an example of an airborne icing conditions sensor with pulsed laser light beams at two different wavelengths (e.g. 905 nm and 1550 nm). When the laser beams are directed into a cloud, the received laser echoes due to backscattering from the droplets are analyzed to estimate the median volume diameter (MVD), the maximum droplet diameter (Dmax), and the liquid water content (LWC) of the cloud. Each laser pulse samples many different droplet sizes at one time and generates a composite backscattered signal. The current scheme for classifying icing clouds is to measure the two color ratio (the ratio of intensities of backscattered light at 905 nm to that at 1550 nm), the optical extinction coefficient at 905 nm, and the lidar ratio (the ratio of the optical extinction to backscatter at 905 nm).
These three measurements work well for classifying clouds with a single mode droplet size distribution. However, droplet size distributions in naturally-occurring clouds are often bimodal due mainly to droplet coalescence. One of the modes occurs at small droplet diameters (˜10 to 20 μm), while larger droplets create a secondary mode at greater than ˜40 μm diameter, depending on the nature of the clouds. Because they are more numerous, small droplets (despite their small areal cross-sections) generate more composite extinction and backscatter than do large droplets. The result is that the extinction, two-color ratio, and the lidar ratio are dominated by scattering from small droplets, even though the fewer large droplets in a bimodal cloud can account for a sizeable fraction of the total liquid water content. Therefore, there is still a need in the art for an improved system and method for determining the presence of large droplets in clouds with bimodal droplet size distributions.